Joint prosthesis

ABSTRACT

A hip joint prosthesis which comprises a cup member for placement in the acetabulum and configurated externally after a substantially spherical surface and internally to form a cavity for snap insertion on the spheroidal head of a stem to which it articulated and which is positioned in medullar cavity of the thigh bone after removal of the thigh bone epiphysis. The pivot center of said spheroidal head of the stem within the cavity of said cup member is located between the center and the spherical outer surface of the cup.

United States Patent Trentani et al. Dec. 2, 1975 JOINT PROSTHESIS 3,728,742 4/1973 Averill etal. 128/92 0 x 3,813,699 6/1974 Giliberty l28/92 C X Invenwrs' 3,829,904 8/1974 Ling ct al 3/1912 fis both of Bologna Primary Examiner-Ronald L. Frinks y Attorney, Agent, or FirmGuid0 Modiano; Albert 22 Filed: Aug. 20, 1974 10511 1211 Appl. No.: 498,883

[57] ABSTRACT [30] Foreign Application Priority Data A hip joint prosthesis which comprises a cup member A 27 1973 h 1 3477/73 for placement in the acetabulum and configurated exa y ternally after a substantially spherical surface and internally to form a cavity for snap insertion on the A i E spheroidal head of a stem to which it articulated and [58] Fie'ld 1913 which is positioned in medullar cavity of the thigh 7 5 A bone after removal of the thigh bone epiphysis. The pivot center of said spheroidal head of the stem within the cavity of said cup member is located between the [56] UNITE S :ZqrENTS center and the spherical outer surface of the cup. 3.722002 3/1973 Charnley 128/92 c x 4 Claims 1 Drawmg Figure US. Patent Dec. 2, 1975 This invention relates to a hip joint prosthesis, i.e. a

prosthesis designed for permanent installation in. a patients body by surgical operation. 1

It is known to use hip joint prosthesis in order to restore an effective degree of deambulatory capabilities in patients affected by processes of the arthrosis type with degeneration and consumption of the joint cartilages or fracture of the thigh bone neck; in both cases the replacement of the thigh bone epiphysis becomes necessary.

The hip prosthesis of conventional design comprises essentially a stem the lower part whereof is inserted and possibly bonded into the medulloid channel of the thigh bone, following the removal of the thigh bone epiphysis, and made to articulate to the acetabulum in the pelvis.

Such known interior surgical appliances may be classed in two groups: a first group comprising a spherical head of solid steel which, being integral with the endomedullary affixing stem, has dimensions compatible with those of the thigh bone head it is conceived to replace, and which articulates directly to the acetabulum, whereas a second group requires replacement of both the joint surfaces, of the thigh bone as well as of the cotyloid cavity; the appliances of the second group differ from the appliances of the first group in that the acetabulum is also involved and covered with a cup of high-density polyethylene, bonded thereto by cementing, thereby the thigh bone component does not rest directly on the articular cartilage of the cotyloid cavity, but instead on the new cotylis of polyethylene.

The prosthesis of the first group involve, due to deambulation and their weight, the wearing out and subsequent breaking of the acetabulum, causing the new epiphysis to penetrate the pelvis within a few years period giving rise to pain, limpness and functional restrictions, whereas for the prosthesis for the second group, which are not subject to the above problem, surgical interventions are required which are more traumatic and thus not recommended, especially when the patients are of old age.

SUMMARY OF THE INVENTION It is an object of the invention to provide a hip joint prosthesis capable to obviate the above difficulties, i.e. prosthesis that is effective in retarding the wearing out and breaking the acetabulum and that does not require surgical operations of a complex and traumatic nature.

Another object of the invention is to provide a hip joint prosthesis having a simple and light structure, easy to manufacture, safe and effective while being of low cost.

According to the present invention a hip joint prosthesis is provided comprising a cup member for placement in an articulating relation in an acetabulum of the pelvis, said cup member having an outer spherical surface and an inner spherical cavity which communicates with the outside through an opening, a femoral insert including a stem for insertion into the femoral bone after removal of the epiphysis thereof and a spheroidal head engaging said spherical cavity through said opening and articulated therein, wherein according to the improvement said spherical cavity is eccentrically located with respect to the outer surface of the cup memher so as to be displaced in a direction diametrically opposite to said opening. g V

DESCRIPTION OF THE DRAWING Further advantages and features of the invention will become more apparent from the following description of a preferred though not exclusive embodiment thereof, and the accompanying exemplary drawing,

wherein: a I v The FIGURE is a partly sectional view of a hip joint prosthesis according to the invention.

DESCRIPTION OE A PREFERRED EMBODIMENT With reference to the drawing, the numeral 1 identifies the cup member of the hip joint prosthesis and the numeral 2 the femoral insert, which will be driven into the diaphysis of the thigh bone: the cup member 1 is advantageously of a plastic material such as thermoplastic polyesters or high density polyehtylene, while for the femoral insert 2 light alloys have proved successful, such as titanium alloys which are oxidation-free.

The cup member 1, has a symmetry of rotation about the axis a and, is shaped so as to have an outer spherical surface whose center is indicated at A. Within the cup member 1 a spherical cavity 4 is provided which communicates externally through an opening 5 which has a frusto-conical inner surface that diverges in a direction outwardly from said cavity and is surrounded by the annular area 3. The center B of the cavity 4 is located on the axis a passing on the center A of the outer surface of the cup member 1.

In the cavity 4, with the aid of a suitable tool, the spheroidal head 6 of the insert 2 can be snap inserted: at connection zone of the head 6, the insert has a reduced cross-section with respect to opening 5 in order to allow for the articulation of the insert within the cavity 4 so that the head 6 is free to rotate in any plane with respect to the endomedullary inphybulum, without however coming apart therefrom.

The lower portion of the insert 1 is constituted by a stem 7, the centerline whereof is b, and which is to be driven and/or bonded into the upper part of the medullary channel of the thigh bone after the removal of the thigh bone epiphysis. The stem 7 is joined to the spheroidal head 6 through an arcuate portion 8: suitably, the arrangement of the insert 2 with respect to cup member 1 will be such as to reproduce as faithfully as possible the angle existing between the dyaphysis and the thigh bone neck. It should be noted that the insert 2 is of a type largely in use for surgical appliances of the second group, and that it is contemplated that the cup member 1 be manufactured in a range of sizes suitable for various applications; with a range of such cup member, since the spheroidal head 6 of the insert has standardized dimensions, it becomes possible, even during the surgical operation the assembling, of hip joint prosthesis adhering as strictly as possible to the characteristics of the thigh bone epiphysis just removed, and this without resorting to a supply of different complete appliances, said supply being costly, or making individually prepared appliances.

Once the stem 7 of the insert has been affixed in the thigh bone dyaphysis, the cup member 1 is placed in cotyloid cavity or acetabulum of the pelvis: thanks to the eccentricity between points A and B, the cup member 1 seats in a position of stable equilibrium since the pivot center B of the head 6 is located between the center A of the outer surface of the cup member and the cotyloid cavity. When deambulating, large relative movements of the head 6 of the stem with respect to the cup member 1 shall correspond to very limited movements of the cupmember l with respect to the cotyloid cavity. Thus, friction between the cup member and the cotyloid cavity is minimized, and wear and breaking of the pelvis are substantially eliminated.

In order to allow for the X-raying of the cup member 1, which is normally of a material impervious to X-rays, close to the area 3 of the head a thin metal wire 9 is provided in a loop configuration.

We claim:

1. Hip-joint prosthesis comprising a cup member for placement in an articulating relation in an acetabulum of the pelvis, said cup member having a continuously smooth outer spherical surface and aninner spherical cavity which communicates with the outside through an opening, a femoral insert including a stem for insertion into the femoral bone after removal of the epiphysis thereof and a spheroidal head engaging said spherical cavity through said opening and articulated therein, wherein according to the improvement said spherical cavity is eccentrically located with respect to the outer surface of the cup member so as to be displaced in a direction diametrically opposite to said opening.

2. Hip-joint prosthesis as defined in claim 1 wherein said opening has an inner frusto-conical surface that diverges in a direction outwardly from said cavity.

3. Hip-joint prosthesis as defined in claim 2 wherein at the connection area of said spheroidal head to the stem, said stem has a lesser cross-section dimension than that of said opening.

4. Hip-joint prosthesis as defined in claim 2 wherein said cup member has a rotation symmetry about an axis passing through the center of said opening and the centers of said outer surface and inner cavity, the center of said cavity being located on said axis between the outer surface and the center thereof. 

1. Hip-joint prosthesis comprising a cup member for placement in an articulating relation in an acetabulum of the pelvis, said cup member having a continuously smooth outer spherical surface and an inner spherical cavity which communicates with the outside through an opening, a femoral insert including a stem for insertion into the femoral bone after removal of the epiphysis thereof and a spheroidal head engaging said spherical cavity through said opening and articulated therein, wherein according to the improvement said spherical cavity is eccentrically located with respect to the outer surface of the cup member so as to be displaced in a direction diametrically opposite to said opening.
 2. Hip-joint prosthesis as defined in claim 1 wherein said opening has an inner frusto-conical surface that diverges in a direction outwardly from said cavity.
 3. Hip-joint prosthesis as defined in claim 2 wherein at the connection area of said spheroidal head to the stem, said stem has a lesser cross-section dimension than that of said opening.
 4. Hip-joint prosthesis as defined in claim 2 wherein said cup member has a rotation symmetry about an axis passing through the center of said opening and the centers of said outer surface and inner cavity, the center of said cavity being located on said axis between the outer surface and the center thereof. 